The liver performs diverse and important metabolic functions in human metabolism, and therefore liver dysfunction can lead to serious health consequences. As a result, many different tests are used to assess liver function. Although these tests provide information concerning the metabolic state of the liver, each procedure has limitations that compromise its value. The galactose breath test represents a non-invasive method of assessing liver function, since galactose is primarily metabolized by the liver. In this test, galactose (labeled with 14C or 13C) is administered to the patient and the appearanCe of label in the expired CO2 is used as an indication of liver function. Although the literature suggests that this test is a good predictor of liver function, due to the radiation risk associated with 14C-galactose and the unavailability of 13C-galactose, the clinical usefulness of the galactose breath test has not been fully evaluated. The goal of this research is to produce 13C-galactose and clinically evaluate the usefulness of this compound as a non-invasive measure of hepatic function. We propose to culture, in the presence of 13CO2, a proprietary microalga that overproduoes galactose. The purified, labeled galactose will then be used for human clinical studies to determine the specificity and sensitivity of this test for assessing liver function.